Coulomb and pinning effects on the soliton-related phonon modes in polyacetylene

The complete phonon spectrum for finite chains of trans-(CH)ₓ is calculated using an extension of the Su-Schrieffer-Heeger model that includes both Hubbard on-site correlations in the meanfield approximation and simple pinning mechanisms appropriate for doped or photoexcited samples. In addition to the extended modes of the optical and acoustic branches we find the four soliton-related modes (T, S, B, and staggered) reported by previous workers who did not, however, include pinning. The relation between these modes and the observed infrared-absorption spectrum is discussed.